In mobile communications, it is common that a user apparatus UE and a base station eNB perform communication so that communication is performed between user apparatuses UE. However, in recent years, various techniques on D2D communication (to be also referred to as device-to-device communication) are being considered for performing direct communication between user apparatuses UE using radio interfaces of LTE.
In the D2D communication technique, the user apparatus UE performs direct communication between user apparatuses UE using a radio resource (a time·frequency resource) used for LTE communication. As D2D communications, for example, there is a communication in which, a user apparatus UE transmits (broadcasts) a discovery signal (discovery signal) including identification information of the user apparatus UE itself, so that another user apparatus UE discovers the user apparatus UE of a communication partner by receiving the discovery signal, and the like.
In a case where the user apparatus UE is within a coverage of a base station eNB, a radio resource used for D2D communication by the user apparatus UE is assigned to the user apparatus UE, for example, as a resource pool for D2D communication from the base station eNB. According to such an assignment, it is possible that cellular communication and D2D communication coexist. In addition, it is being studied to enable D2D communication even when the user apparatus UE is out of coverage of the base station eNB.